The present invention relates to method and apparatus for cancelling an echo in a system having a plurality of received signals and a single or a plurality of transmission signal or signals.
Regarding multi-channel echo cancelling method and apparatus for cancelling an echo occurring by transmission of the received signal through a spatial acoustic path in the system having a plurality of received signals and a single or a plurality of transmission signal or signals, there have been proposed two types of systems, such as a cascade connection type and a linear combination type, in the technical Report of the Institute of Electronics, Information and Communication Engineers (IEICE) of Japan Vol. 84, No. 330, pp. 714, CS-84-714 (hereafter, referred as Reference 1). According to the Reference 1, since the cascade connection type has a restriction of a constitution, an echo suppression performance thereof is inferior to that of the linear combination type. Accordingly, there will be described a case where a linear combination type multi-channel echo cancelling apparatus (an echo canceller) is applied to a two channel system having a pair of both reception and transmission signals.
FIG. 24 shows a linear combination type multi-channel echo canceller. A first received signal 1 is reproduced by a first speaker 3 and is going through a spatial acoustic path to a first microphone 9 so as to generate a first echo 5. A second received signal 2 is reproduced by a second speaker 4 and is going through a spatial acoustic path to the first microphone 9 so as to generate a second echo 6. A first mixed signal 14 is generated by adding the first and second echoes 5 and 6 and a first transmission signal 12 which is inputted to the first microphone 9 corresponding to a voice of a talker 11. As the same manner, the first received signal 1 is reproduced by the first speaker 3 and is going through the spatial acoustic path to a second microphone 10 so as to generate a third echo 7. The second received signal 2 is reproduced by the second speaker 4 and is going through a spatial acoustic path to the second microphone 10 so as to generate a fourth echo 8. A second mixed signal 15 is generated by adding the third and fourth echoes 7 and 8 and a second transmission signal 13 which is inputted to the second microphone 10 corresponding to a voice of the talker 11.
In order to cancel an echo which is mixed in the first mixed signal 14, an echo replica 125 corresponding to the first echo 5 is generated by inputting the first received signal 1 in a first adaptive filter 121, and an echo replica 126 corresponding to the second echo 6 is generated by inputting the second received signal 2 in a second adaptive filter 122. A first subtracter 129 subtracts the echo replicas 125 and 126 respectively corresponding to the first and second echoes 5 and 6 from the first mixed signal 14. The first and second adaptive filters 121 and 122 are controlled such that the first subtracter 129 has the minimum output. An output of the first subtracter 129 is a first output signal 16 of an echo canceller 120.
In order to cancel an echo which is mixed in the second mixed signal 15, an echo replica 127 corresponding to the third echo 7 is generated by inputting the first received signal 1 in a third adaptive filter 123, and an echo replica 128 corresponding to the fourth echo 8 is generated by inputting the second received signal 2 in a fourth adaptive filter 124. A second subtracter 130 subtracts the echo replicas 127 and 128 respectively corresponding to the third and fourth echoes 7 and 8 from the second mixed signal 15. The third and fourth adaptive filters 123 and 124 are controlled such that the second subtracter 130 to has the minimum output. An output of the second subtracter 130 is a second output signal 17 of the echo canceller 120.
In a multi-channel television conference system as one of the main applications of multi-channel echo cancellers, since the voice of a talker is recorded by a plurality of microphones, the received signal recorded by each microphone may be approximated to have an attenuation and a delay corresponding to a distance between the talker and the microphone compared to the other received signal. Accordingly, inter-channel correlation of the received signals becomes high.
It this application, a second received signal 2 which is a delayed version of the first received signal 1, an echo path which can be modeled as an FIR (Filter impulse response) filter and an echo canceller based on linear combination are assumed.
The first and second received signals 1 and 2 at the time n denoted as x1(n) and x2(n), and an echo which is mixed in the first mixed signal 14 as d(n). When a time difference between the first and the second received signals is nd (a natural number) samples, equation (1) can be obtained:
X2(n)=X1(nxe2x88x92nd)xe2x80x83xe2x80x83(1) 
For simplicity, it is assumed that the entire spatial acoustic paths from the first and the second speakers 3 and 4 to the first and second microphones 9 and 10 have the same length N for its impulse response. Further, a symbol h1,i denotes an impulse response sample of the acoustic path from the speaker 3 to the microphone 9, and a symbol h2,i denotes an impulse response sample of the acoustic path from the speaker 4 to the microphone 9. Here, i is an integer between 0 and Nxe2x88x921. The echo d(n) which is mixed in the mixed signal 14 can be obtained as a sum of the echoes 5 and 6 according to a equation (2) as follows:                               d          ⁡                      (            n            )                          =                                            ∑                              i                =                0                                            N                -                1                                      ⁢                                          h                                  1                  ,                  i                                            ⁢                                                x                  1                                ⁡                                  (                                      n                    -                    i                                    )                                                              +                                    ∑                              i                =                0                                            N                -                1                                      ⁢                                          h                                  2                  ,                  i                                            ⁢                                                x                  2                                ⁡                                  (                                      n                    -                    i                                    )                                                                                        (        2        )            
When equation (1) is combined with equation (2) to eliminate x2(n), equation (3) can be obtained as follows:                               d          ⁡                      (            n            )                          =                                            ∑                              i                =                0                                            nd                -                1                                      ⁢                                          h                                  1                  ,                  i                                            ⁢                                                x                  1                                ⁡                                  (                                      n                    -                    i                                    )                                                              +                                    ∑                              i                =                nd                                            N                -                1                                      ⁢                                          (                                                      h                                          1                      ,                      i                                                        +                                      h                                          2                      ,                                              i                        -                        nd                                                                                            )                            ⁢                                                x                  1                                ⁡                                  (                                      n                    -                    i                                    )                                                              +                                    ∑                              i                =                                  N                  -                  nd                                                            N                -                1                                      ⁢                                          h                                  2                  ,                  i                                            ⁢                                                x                  1                                ⁡                                  (                                      n                    -                                          n                      d                                        -                    i                                    )                                                                                        (        3        )            
If the i-th filter coefficient of the adaptive filters 121 and 122 are respectively denoted as w1,i(n) and w2,i(n), the echoe replica d {circumflex over ( )}(n) (d(n) hat({circumflex over ( )}) ) which is generated by the adaptive filters 121 and 122, can be obtained by a equation (4) as follows:                                           d            ^                    ⁡                      (            n            )                          =                                            ∑                              i                =                0                                            N                -                1                                      ⁢                                                            w                                      1                    ,                    i                                                  ⁡                                  (                  n                  )                                            ⁢                                                x                  1                                ⁡                                  (                                      n                    -                    i                                    )                                                              +                                    ∑                              i                =                0                                            N                -                1                                      ⁢                                                            w                                      2                    ,                    i                                                  ⁡                                  (                  n                  )                                            ⁢                                                x                  2                                ⁡                                  (                                      n                    -                    i                                    )                                                                                        (        4        )            
When the equation (1) combined with equation (4) to eliminate x2(n), it is possible to obtain equation (5) as follows:                                           d            ^                    ⁡                      (            n            )                          =                                            ∑                              i                =                0                                            nd                -                1                                      ⁢                                                            w                                      1                    ,                    i                                                  ⁡                                  (                  n                  )                                            ⁢                                                x                  1                                ⁡                                  (                                      n                    -                    i                                    )                                                              +                                    ∑                              i                =                nd                                            N                -                1                                      ⁢                                          {                                                                            w                                              1                        ,                        i                                                              ⁡                                          (                      n                      )                                                        +                                                            w                                              2                        ,                                                  i                          -                          nd                                                                                      ⁡                                          (                      n                      )                                                                      }                            ⁢                                                x                  1                                ⁡                                  (                                      n                    -                    i                                    )                                                              +                                    ∑                              i                =                                  N                  -                  nd                                                            N                -                1                                      ⁢                                                            w                                      2                    ,                    i                                                  ⁡                                  (                  n                  )                                            ⁢                                                x                  1                                ⁡                                  (                                      n                    -                                          n                      d                                        -                    i                                    )                                                                                        (        5        )            
A redidual echo e(n) can be obtained by equation (6) as follows:                               e          ⁡                      (            n            )                          =                                            ∑                              i                =                0                                            nd                -                1                                      ⁢                                          {                                                      h                                          1                      ,                      i                                                        -                                                            w                                              1                        ,                        i                                                              ⁡                                          (                      n                      )                                                                      }                            ⁢                                                x                  1                                ⁡                                  (                                      n                    -                    i                                    )                                                              +                                    ∑                              i                =                nd                                            N                -                1                                      ⁢                                          {                                                      h                                          1                      ,                      i                                                        +                                      h                                          2                      ,                                              i                        -                        nd                                                                              -                                                            w                                              1                        ,                        i                                                              ⁡                                          (                      n                      )                                                        -                                                            w                                              2                        ,                                                  i                          -                          nd                                                                                      ⁡                                          (                      n                      )                                                                      }                            ⁢                                                x                  1                                ⁡                                  (                                      n                    -                    i                                    )                                                              +                                    ∑                              i                =                                  N                  -                  nd                                                            N                -                1                                      ⁢                                          {                                                      h                                          2                      ,                      i                                                        -                                                            w                                              2                        ,                        i                                                              ⁡                                          (                      n                      )                                                                      }                            ⁢                                                x                  1                                ⁡                                  (                                      n                    -                                          n                      d                                        -                    i                                    )                                                                                        (        6        )            
To completely cancel the echo, the following conditions must be satisfied:                                                                         h                                  1                  ,                  i                                            =                                                w                                      1                    ,                    i                                                  ⁡                                  (                  n                  )                                                                                                        i                =                0                            ,              ⋯              ⁢                              xe2x80x83                            ,                                                n                  d                                -                1                                                                                                                          h                                      1                    ,                    i                                                  +                                  h                                      2                    ,                                          i                      -                      nd                                                                                  =                                                                    w                                          1                      ,                      i                                                        ⁡                                      (                    n                    )                                                  +                                                      w                                          2                      ,                                              i                        -                        nd                                                                              ⁡                                      (                    n                    )                                                                                                                          i                =                                  n                  d                                            ,              ⋯              ⁢                              xe2x80x83                            ,                              N                -                1                                                                                                        h                                  2                  ,                  i                                            =                                                w                                      2                    ,                    i                                                  ⁡                                  (                  n                  )                                                                                                        i                =                                  N                  -                                      n                    d                                                              ,              ⋯              ⁢                              xe2x80x83                            ,                              N                -                1                                                                        (        7        )            
According to a equation (7),
w1,0(n), . . . , w1,ndxe2x88x921 (n) and w2,Nxe2x88x92nd (n), . . . , w2,Nxe2x88x921 (n) 
is uniquely determined, however solutions to
w1,nd (n), . . . , w1,Nxe2x88x921 (n) and w2,0 , . . . , w2,Nxe2x88x92ndxe2x88x921 (n) 
include an infinite number of combinations. Specifically, since solutions to
W1,nd (n), . . . , W1,Nxe2x88x921 (n) and W2,0 , . . . , W2,Nxe2x88x92ndxe2x88x921 (n) 
depend on the value of nd, therefore, when the value of nd changes with a movement of the talker, the solutions change there with. This means that an echo cancellation capability deteriorates even in a case where the echo path does not change, so as to result in an obstruction in an actual use. As described above, even though the explanation has been performed with respect only to the adaptive filters 121 and 122 used for cancelling an echo mixed in the mixed signal 14, the same explanation may be established with respect to the adaptive filters 123 and 124.
In order to solve this problem, a multi-channel echo cancelling apparatus, in which a single adaptive filter per channel cancels an echo which is generated by the sum of signals propagated from one sound source through plurality of paths by generating echo replicas with adaptive filters corresponding one to one to the mixed signals, is disclosed in IEEE Proceedings of International Conference on Acoustics, Speech and Signal Processing Vol. 2, 1994, p.p. 245-248 (hereafter, referred to as Reference 2).
In the multi-channel echo cancelling apparatus disclosed in Reference 2, the solution does not become indefinite, because each adaptive filter cancels the echo occurring in the corresponding channel. Accordingly, coefficients of the adaptive filters converge to the optimum values that are uniquely defined. However, in the Reference 2, it is described as an evaluation result that the echo cancellation capability deteriorates when parameters determined by the used environment such as the arrangement of the microphones to record the talker voice are not within a certain range. Accordingly, in order to use the cancellation apparatus in a variety of environment, a multi-channel echo canceller based on linear combination must be used.
On the basis of the above premise, a system capable of uniquely identifying coefficients of the adaptive filter has been proposed. This system is a multi-channel echo canceller based on linear combination which generates a delayed signal from the received signal, and utilize this delayed signal as new received signal by periodically alternating it with the original received signal. The system is disclosed in the Technical Report of the Institute of Electronics, and Information and Communication Engineers (IEICE) of Japan (hereafter, referred as Reference 3). In the multi-channel echo cancellation system disclosed in the Reference 3, since a number of equations, which are used for calculating coefficients of the adaptive filters, increases by introducing the delayed received signal, it is does not have a problem of the indefinite number of solutions. Accordingly, the coefficients of the adaptive filter converge to the optimum values which are uniquely determined. However, the Reference 3 also discloses that this system has a problem that switching between the received signal and the delayed received signal causes aliasing, which leads to inferior sound quality.
As has been described so far by using FIG. 24, the conventional multi-channel echo cancellation method and apparatus have the problem that the coefficients of the adaptive filter have an indefinite number of and that the adaptive filter can not reach the solution that is uniquely determined by the impulse response of the echo path. Further, the system that is proposed by the Reference 3 could not avoid deterioration of the sound quality by aliasing. The objective of the present invention is to provide a multi-channel echo cancellation method and apparatus having coefficient values that converge to the true values which are uniquely determined by the impulse response of the echo path, so as to have an excellent sound quality.
The objective of the present invention is to provide a method and apparatus for cancelling multi-channel echoes, in which coefficient values of the adaptive filter converge to the true values which are uniquely determined by the impulse response of an echo path to achieve excellent sound quality.
A multi-channel echo cancellation method and apparatus according to the present invention first generate a supplemental signal by filtering one of the received signals and second generate a new received signal by switching between over the received signal and the supplemental signal. Further, a cycle of the changeover is set to be longer than the sampling period of the received signal.
In detail, the system has a filter (145 in FIG. 1) for generating a supplemental signal by processing one of the received signals, a switch (141 in FIG. 1) for switching between the input and the output of the filter to generate a new received signal, and a frequency divider (143 in FIG. 1) for generating a change-over timing signal of the switch.
Further, the multi-channel echo cancellation method and apparatus according to the present invention first generate a supplemental signal by filtering one of the received signals and second generate a new received signal by switching between the received signal and the supplemental signal. Further, a cycle of the changeover is set to be longer than the sampling period of the received signal, and the changeover is performed corresponding to the received signal characteristics.
In detail, the system has a filter (145 in FIG. 4) for generating a supplemental signal by processing one of the received signals, a switch (141 in FIG. 4) for switching between input and the output of the filter to generate a new received signal, a frequency divider (143 in FIG. 4) for generating a changeover timing signal of the switch, an analysis circuit (147 in FIG. 4) for analyzing the received signal, and a logical multiplier (146 in FIG. 4) for detecting a coincidence of the changeover timing signal and the timing signal after analyzing the received signal, so that an output of the logical multiplier changes over the switch.
Furthermore, the multi-channel echo cancellation method and apparatus according to the present invention first generate a supplemental signal by filtering one of received signals and second generate a new received signal by switching between the received signal and the supplemental signal. The changeover is performed by a changeover signal that is generated on the basis of the analyzed result of the received signal.
In detail, the system has a filter (145 in FIG. 7) for generating a supplemental signal by filtering one of the received signals, a switch (141 in FIG. 7) for switching between the input and the output of the filter to generate the new received signal, and an analysis circuit (148 in FIG. 7) for generating a changeover signal of the switch under the consideration of a changeover cycle after analyzing the received signal.
The multi-channel echo cancellation method and apparatus according to the present invention generates a supplemental signal after filtering one of the received signals, switching between the original received signal and the supplemental signal, and drives the adaptive filter by the new received signal periodically switching between the signals. Since a plurality of adaptive filters estimate the echo generated by transmission from one signal source through a plurality of paths, it is possible to increase the number of the conditions for obtaining the adaptive filter coefficients, so that there is no problem that the number of solutions becomes indefinite. Accordingly, the coefficients of the adaptive filter converge to the optimum values uniquely determined. Further, since the timing and period of the switching between the original and the supplemental signals are controlled on the basis of the characteristics of the received signals, it is possible to suppress the deterioration of the quality of the received signals that are directly supplied to the speakers and heard by listeners, thereby keeping on excellent sound quality.
A multi-channel echo cancellation apparatus according to the present invention uses a signal processed from one of the received signals as the received signal.
In detail, the apparatus comprises a pre-processing circuit (200 in FIG. 8) for pre-processing the received signal 2 and supplying it to adaptive filters 122 and 124 and digital/analog converter (DAC) 19.
Further, the multi-channel echo cancellation apparatus according to the present invention uses a new received signal, which is generated by processing one of the original received signals, and at the same time, modifies the amplitude of the other received signal.
In detail, the apparatus comprises a pre-processing circuit (300 in FIG. 20) for pre-processing the received signal 2 and supplying it to adaptive filters 122 and 124 and digital/analog converter 19, and an amplitude modification circuit (400 in FIG. 20) for modifying the amplitude of the received signal 1 and for supplying its output signal to adaptive filters 121 and 123 and digital/analog converter 18.
A multi-channel echo cancellation apparatus according to the present invention generates a supplemental signal after filtering one of the received signals, and drives the adaptive filters by the new received signal, which is obtained as a multiplexed signal of the original received signal and the newly generated supplemental signal. Since a plurality of adaptive filters estimate the echo generated by a plurality of transmission paths from one sound source, the number of conditions for obtaining the adaptive filter coefficients increase, so that it is possible to eliminate the problem that the number of solutions becomes indefinite. Accordingly, the coefficients of the adaptive filter converge to the optimum value uniquely defined.
Further, the multi-channel echo cancellation apparatus controls parameters for multiplexing the original received signal and the supplemental signal based on the characteristics of the received signal, and at the same time, offsets a sound image shift caused by the use of the supplemental signal by means of an amplitude modification for the input signal. Accordingly, it is possible to keep excellent sound quality by suppressing quality deterioration of the received signal directly supplied to the speaker for listening.